Many factors affect gene expression in organisms. For instance, small RNAs, generally 20 to 25 nucleotides in length, have emerged as important regulators of eukaryotic gene expression. One class of small RNA is the short interfering RNA (siRNA). siRNA plays a role in the RNA interference (RNAi) pathway, and particularly in RNA silencing, a sequence-specific RNA degradation process that is triggered by double-stranded RNA (dsRNA). siRNAs are double-stranded with small 3′ overhangs and derive from longer dsRNA precursors that induce silencing. They serve as guides to direct destruction of target RNA and have been implicated as primers in the amplification of dsRNA via the activity of a cellular RNA dependent RNA polymerase.
Since their discovery, synthetic siRNA have been produced that may induce RNAi in mammalian cells through silencing or otherwise suppressing transcription of multiple genes. However, despite promising results, problems still exist with successful utilization of siRNA technology, among which delivery methods play a large role. Typically, siRNA have been delivered either by direct injection, electroproration, or by complexing with a transfecting agent. However, the siRNA remains actively present for only a matter of hours following delivery. In order to obtain longer lasting effectiveness, improved delivery methods must be found that may provide steady, long-term delivery of siRNA.
Transdermal delivery devices that provide a route for siRNA to be delivered in an active state at effective, steady concentrations over a period of time would be of great benefit. Many difficulties must be overcome to reach this goal. For instance, the human body has developed many systems to prevent the influx of foreign substances such as enzymatic degradation in the gastrointestinal tract, structural components that prevent absorption across epithelium, hepatic clearance, and immune and foreign body response.
Transdermal devices have been developed for sustained delivery of certain drugs including those for treatment of vertigo, contraception, and smoking addiction. In order to be successful, a transdermal device must deliver an agent across the epidermis, which has evolved with a primary function of keeping foreign substances out. The outermost layer of the epidermis, the stratum corneum, has structural stability provided by overlapping corneocytes and crosslinked keratin fibers held together by coreodesmosomes and embedded within a lipid matrix, all of which provide an excellent barrier function. Beneath the stratum corneum is the stratum granulosum, within which tight junctions are formed between keratinocytes. Tight junctions are barrier structures that include a network of transmembrane proteins embedded in adjacent plasma membranes (e.g., claudins, occludin, and junctional adhesion molecules) as well as multiple plaque proteins (e.g., ZO-1, ZO-2, ZO-3, cingulin, symplekin). Tight junctions are found in internal epithelium (e.g., the intestinal epithelium, the blood-brain barrier) as well as in the stratum granulosum of the skin. Beneath both the stratum corneum and the stratum granulosum lays the stratum spinosum. The stratum spinosum includes Langerhans cells, which are dendritic cells that may become fully functioning antigen-presenting cells and may institute an immune response and/or a foreign body response to an invading agent.
Unfortunately, transdermal delivery methods are presently limited to delivery of low molecular weight agents that have a moderate lipophilicity and no charge. Even upon successful crossing of the natural boundary, problems still exist with regard to maintaining the activity level of delivered agents and avoidance of foreign body and immune response.
The utilization of supplementary methods to facilitate transdermal delivery of active agents has improved this delivery route. For instance, microneedle devices have been found to be useful in transport of material into or across the skin. In general, a microneedle device includes an array of needles that may penetrate the stratum corneum of the skin and reach an underlying layer. Examples of microneedle devices have been described in U.S. Pat. No. 6,334,856 to Allen, et al. and U.S. Pat. No. 7,226,439 to Prausnitz, et al., both of which are incorporated herein by reference.
While the above describes improvement in the art, room for further improvement exists.